Domestic wastewater causes nitrate pollution in an agricultural watershed, China

Excessive quantities of nitrates in the aquatic environment can cause eutrophication and raise water safety concerns. Therefore, identification of the sources of nitrate is crucial to mitigate nitrate pollution and for better management of the water resources. Here, the spatiotemporal variations and sources of nitrate were investigated by stable isotopes (delta N-15 and delta O-18), hydrogeochemical variables (e.g., NO3- and Cl-), and exogenous microbial signals (i.e., sediments, soils, domestic and swine sewage) in an agricultural watershed (Changle River watershed) in China. The concentration ranges of delta N-15 and delta O-18-NO3- between 3.03 parts per thousand-18.97 parts per thousand and -1.55 parts per thousand-16.47 parts per thousand, respectively, suggested that soil nitrogen, chemical fertilizers, and manure and sewage (M&S) were the primary nitrate sources. Bayesian isotopic mixing model suggested that the major proportion of nitrate within the watershed (53.12 +/- 10.40% and 63.81 +/- 15.08%) and tributaries (64.43 +/- 5.03% and 76.20 +/- 4.34%) were contributed by M&S in dry and wet seasons, respectively. Community-based microbial source tracking (MST) showed that untreated and treated domestic wastewater was the major source (>70%) of river microbiota. Redundancy analysis with the incorporation of land use, hydrogeochemical variables, dual stable isotope, and exogenous microbial signals revealed domestic wastewater as the dominant cause of nitrate pollution. Altogether, this study not only identifies and quantifies the spatiotemporal variations in nitrate sources in the study area but also provides a new analytical framework by combining nitrate isotopic signatures and community-based MST approaches for source appointment of nitrate in other polluted watersheds.

Automated summary

This study investigates the spatiotemporal variations and sources of nitrate in an agricultural watershed in China using stable isotopes, hydrogeochemical variables, and exogenous microbial signals. The results suggest that soil nitrogen, chemical fertilizers, and manure and sewage are the primary sources of nitrate, with domestic wastewater being the dominant cause of nitrate pollution.